xref: /dragonfly/lib/libc/string/strlcpy.3 (revision b40e316c) 
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3.\" Copyright (c) 1998 Todd C. Miller <Todd.Miller@courtesan.com>
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28.\" $FreeBSD: src/lib/libc/string/strlcpy.3,v 1.4.2.8 2002/01/19 12:29:40 yar Exp $
29.\" $DragonFly: src/lib/libc/string/strlcpy.3,v 1.2 2003/06/17 04:26:46 dillon Exp $
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31.Dd June 22, 1998
32.Dt STRLCPY 3
33.Os
34.Sh NAME
35.Nm strlcpy ,
36.Nm strlcat
37.Nd size-bounded string copying and concatenation
38.Sh LIBRARY
39.Lb libc
40.Sh SYNOPSIS
41.In string.h
42.Ft size_t
43.Fn strlcpy "char *dst" "const char *src" "size_t size"
44.Ft size_t
45.Fn strlcat "char *dst" "const char *src" "size_t size"
46.Sh DESCRIPTION
47The
48.Fn strlcpy
49and
50.Fn strlcat
51functions copy and concatenate strings respectively.  They are designed
52to be safer, more consistent, and less error prone replacements for
53.Xr strncpy 3
54and
55.Xr strncat 3 .
56Unlike those functions,
57.Fn strlcpy
58and
59.Fn strlcat
60take the full size of the buffer (not just the length) and guarantee to
61NUL-terminate the result (as long as
62.Fa size
63is larger than 0 or, in the case of
64.Fn strlcat ,
65as long as there is at least one byte free in
66.Fa dst ) .
67Note that you should include a byte for the NUL in
68.Fa size .
69Also note that
70.Fn strlcpy
71and
72.Fn strlcat
73only operate on true
74.Dq C
75strings.
76This means that for
77.Fn strlcpy
78.Fa src
79must be NUL-terminated and for
80.Fn strlcat
81both
82.Fa src
83and
84.Fa dst
85must be NUL-terminated.
86.Pp
87The
88.Fn strlcpy
89function copies up to
90.Fa size
91- 1 characters from the NUL-terminated string
92.Fa src
93to
94.Fa dst ,
95NUL-terminating the result.
96.Pp
97The
98.Fn strlcat
99function appends the NUL-terminated string
100.Fa src
101to the end of
102.Fa dst .
103It will append at most
104.Fa size
105- strlen(dst) - 1 bytes, NUL-terminating the result.
106.Sh RETURN VALUES
107The
108.Fn strlcpy
109and
110.Fn strlcat
111functions return the total length of the string they tried to
112create.  For
113.Fn strlcpy
114that means the length of
115.Fa src .
116For
117.Fn strlcat
118that means the initial length of
119.Fa dst
120plus
121the length of
122.Fa src .
123While this may seem somewhat confusing it was done to make
124truncation detection simple.
125.Pp
126Note however, that if
127.Fn strlcat
128traverses
129.Fa size
130characters without finding a NUL, the length of the string is considered
131to be
132.Fa size
133and the destination string will not be NUL-terminated (since there was
134no space for the NUL).
135This keeps
136.Fn strlcat
137from running off the end of a string.
138In practice this should not happen (as it means that either
139.Fa size
140is incorrect or that
141.Fa dst
142is not a proper
143.Dq C
144string).
145The check exists to prevent potential security problems in incorrect code.
146.Sh EXAMPLES
147The following code fragment illustrates the simple case:
148.Bd -literal -offset indent
149char *s, *p, buf[BUFSIZ];
150
151\&...
152
153(void)strlcpy(buf, s, sizeof(buf));
154(void)strlcat(buf, p, sizeof(buf));
155.Ed
156.Pp
157To detect truncation, perhaps while building a pathname, something
158like the following might be used:
159.Bd -literal -offset indent
160char *dir, *file, pname[MAXPATHLEN];
161
162\&...
163
164if (strlcpy(pname, dir, sizeof(pname)) >= sizeof(pname))
165	goto toolong;
166if (strlcat(pname, file, sizeof(pname)) >= sizeof(pname))
167	goto toolong;
168.Ed
169.Pp
170Since we know how many characters we copied the first time, we can
171speed things up a bit by using a copy instead of an append:
172.Bd -literal -offset indent
173char *dir, *file, pname[MAXPATHLEN];
174size_t n;
175
176\&...
177
178n = strlcpy(pname, dir, sizeof(pname));
179if (n >= sizeof(pname))
180	goto toolong;
181if (strlcpy(pname + n, file, sizeof(pname) - n) >= sizeof(pname) - n)
182	goto toolong;
183.Ed
184.Pp
185However, one may question the validity of such optimizations, as they
186defeat the whole purpose of
187.Fn strlcpy
188and
189.Fn strlcat .
190As a matter of fact, the first version of this manual page got it wrong.
191.Sh SEE ALSO
192.Xr snprintf 3 ,
193.Xr strncat 3 ,
194.Xr strncpy 3
195.Sh HISTORY
196.Fn strlcpy
197and
198.Fn strlcat
199functions first appeared in
200.Ox 2.4 ,
201and made their appearance in
202.Fx 3.3 .
203